Electrolytic production of tungsten and molybdenum



United States Patent M 3,297,553 ELECTROLYTIC PRODUCTION OF TUNGSTEN AND MOLYBDENUM Harvey L. Slatin, New York, N.Y., assignor to Timax sstliciates, New York, N.Y., apartnership of New or No Drawing. Filed May 28, 1963, Ser. No. 283,697 6 Claims. (Cl. 204-64) This invention relates to the production of tungsten and molybdenum directly from crude concentrates by fusion electrolysis. v

In my Patent No. 2,960,451 entitled, electrolytic Production of Refractory Multivalent Metals, a method for producing high purity tungsten or molybdenum is disclosed by electrolyzing tungsten anhydride (W0 or molybdic oxide dissolved in a fused bath containing alkaline earth metal halides, such as calcium chloride (CaCI and alkaline earthijmetal oxides, such as icalcium oxide (CaO). When scheelite (CaWO or .powellite (CaMoO is used in place of the tungsten anhydride or molybdic oxide as feed material, no calcium oxide is required since scheelite and powellite already contain the required calcium oxide.

In carrying out this latter embodiment of the process, the crude scheelite, powellite or processed molybdenum ores may be supplied directly to the electrolyte with a tungsten or molybdenum deposit of over 99% purity being obtained from the electrolyte. The metallic contaminants, such as iron or molybdenum which are usually in high concentration in the crude scheelite concentrate feed, appear in minor traces or in greatly reduced proportion in the deposit.

In continuous processing the concentration of calcium oxide in the electrolyte builds up as more and more scheelite or processed powellite concentrate is added to replenish the depleted tungsten or molybdenum. As the concentration of excess calcium oxide increases in the electrolyte, the quality of the electrodeposition process seems to be impaired so that the recovery efiiciency falls off and the purity of the deposit decreases. As described in my Patent No. 2,960,451 the concentration of the alkaline earth metal oxide may be in the range from below 5 weight percent to 40 and over with efiective results. The preferred concentrations are in the order of -20% by Weight and are related to the W0 concentration.

The primary object of this invention is to improve the process so that consistently high purity tungsten or molybdenum may be produced with high recovery efiiciency in the fusion electrolysis of crude ore concentrate and calcium chloride electrolyte.

Another object of this invention is to provide a means for maintaining the electrolyte in such condition that the crystal size of the deposited tungsten or molybdenum remains large; the color of the deposit is silvery; the metal is substantially free of bronzes, lower oxides, carbides, borides, phosphides, silicides, sulfides, selenides, arsenides or the like.

These and other objects are realized by the invention by periodically converting the excess calcium oxide which builds up in the electrolyte to calcium chloride, which is the principal ingredient of the bath. This purpose may be attained in many ways, but it is preferable to bubble chlorine gas through the bath. By doing this periodically either before or after or during the addition of the ore, the electrolyte is sustained in such condition so as to maintain continued operation of the process.

The term high purity metal as used herein refers to metal whose purity lies above 99%.

The improvement embodied by the instant invention may better be understood by considering the following illustrations of my invention.

The electrolytic cell used in the following examples 3,297,553 Patented Jan. 10, 1967 was made of graphite set in a welded shell crucible made of Inconel. The cell was 10" OD. and 8" LD. and 14" tall, inside height. The cell was not sealed airtight but the cathode electrodeposit was allowed to cool in a cylinder surrounding the cathode through which helium gas was passed, thus effectively isolating the cathode from air and other gaseous contaminants. The electrolytic cell was made the anode to a depended cathode centrally located in the graphite crucible. The fused salt depth was about 10 inches with about 7" of cathode immersion. The cathode in these runs was an AGS-X grade graphite rod having an outside diameter of two inches and a length of three feet.

The cell assembly was placed in an electrically heated furnace which was thermostatically controlled. The direct current was supplied by a voltage adjustable rectifier.

The raw material feed was crude mill concentrate scheelite having the following petrographic analysis:

Vol. percent Pyrrohotite Garnet Actinolite Epidote Chloride Scheelite Chemical analysis:

Percent by wt.

The solute was anhydrous calcium chloride weighing about 33 lbs. which was fused to a clear water-white melt. Suflicient scheelite concentrate was added to provide a 5 to 10% by weight solution calculated as tungsten. This concentration appeared to give the best results. At higher concentrations scheelite crystallizes out and may interfere physically. The bath depth is about 10'.

The operating procedure is as follows. A 2" OD. graphite rod, acting as cathode, is lowered into the molten bath for an immersion of seven inches. The current is adjusted to the selected cathode current density based on the 2 OD. X 7 immersion. A cathode that has been previously used in the electrolysis of tungsten is preferred, since in such use a thin film is produced which adheres tenaciously to the electrode and facilitates the adherence of subsequent electrodeposits.

The electrolysis is continued at the temperature set for 4 hours. Then the cathode with its electrodeposit and still under impressed D.C. voltage is slowly withdrawn from the bath and retained in the cell. The fiuid salts are allowed to drain back into the bath. During this time a clamshell-like steel cylinder surrounds the cathode through which helium or other inert gas may be passed to form an inert atmosphere around the deposit.

After the cathode has cooled sufliciently to be handled, it is detached from the cathode holder and another electrode is put in its place. The electrolysis is recommenced.

The electrodeposit is stripped from the cathode and introduced into a container having a mechanical agitator such as a Lightning stirrer and a water inlet and weir overflow. In a few minutes the electrodeposi-ted mass disintegrates as the soluble salts are washed away. Since the cell is not airtight, carbonates may form in the electrolyte. These are easily decomposed by HCl solution to facilitate the washin Further aid to cleaning up the metal deposit may comprise treating with caustic soda solution and the like. When the metal is saltfree by test for chloride ion, the deposited metal is transferred to a filter and air dried.

The dried metal is silvery in color. It is classified as to particle size and analyzed for blending or other desired purpose.

TAB LE I Duration of Run: 4 hrs. each (2 runs mixed). Temperature of Electrolyte: 1,000 G.

Bath Conc.: W in 021012 (teed schcelite concentrate). Voltage: 2.4 V.

Current: 150 amps.

Cathode current density: 3.5 amps./in.'- initial.

Screen Analysis:

Size

Wt. Chem. Analysis, W 95.5 9 Major Impurities Brick, Fe, C Brick, Fe, (3.. Bio .019 M0 .019

Fe .5 Fe .5 X-ray Impurities .Ol W C"--. .01 \V C None None.

.3 schcelite. .2 scheclite Each day or as required five pounds or more of scheelite concentrate is added to the electrolyte to replenish the depleted tungsten. I have found that by blowing dry chlorine gas through an inert tube made of quartz or Sillimanite towards the bottom of the cell at a rate so that the bath is well stirred by the gas bubbles and The brick originated from improper handling of the furnace cover. The carbon appears through mechanical abrasion and the scheelite through mechanical entrainment. Much of the iron may be attributed to corrosion of the shell linin With reasonable care and proper cell design, such extraneous impurities do not appear,

TABLE II Duration 01 Run: 4 hrs. each (2 runs mixed). Cathode: 2" Dia. graphite rod 7 immersion. Temperature of Electrolyte: 1.000 C.

Bath: 10 depth containing 10% by wt. W in 02.012. Anode: Cell crucible (graphite).

Voltage: 2.2 volts.

Current: 150 amperes.

Cathode current density: 3.5 amps/in. initial calc.

Screen Analysis:

Size +100 +150 +200 +325 325 Wt. Z, 3. 4 10. 9 15.7 43. 5 26. 4 Chem. Analysis, Wt. Per- I cent W 96. 2 98. 8 99. 1 98. 8 98. 6 Principal Impurity Fe, brick Fe Fe Fe Fe X-ray Impurities None None None None None continuing the flow until free chlorine gas is detected in the vented gas, the quality of the electrodeposit is maintained and the high recovery efliciency is sustained. This procedure is followed each day after the scheelite addition.

No scheelite or tungsten carbide was found in these samples. Iron is heaviest in mesh, decreasing in the finer meshes to the lowest amount in -325 mesh. The iron is not in excess of /2%. Also molybdenum was below 0.01%.

TABLE III Duration of Run: 4 hrs. each (2 runs mixed). Temperature of Electrolysis: 1,000 0.

Bath C0nc.: 9" depth, 6% by wt. calc. as W in 011013. Cathode: 2 dia. graphite 6" immersion.

Anode: Graphite cell pot.

Voltage: 2.2 volts.

Current: amperes.

Cathode current density: 4.0 amps/in. cale.

Screen Analysis:

Size +100 +150 +200 +325 325 Wt. 6.7 12. 1 19. 0 38.8 23. 4 Chem. Analysis W in Wt.

Percent- 94. 5 98. 5 98. 8 99. 0 99. 4 Impurities Brick, Fe F Fe Fe Fe X-ray Analysis Not Taken TABLE IV Duration of Run: 4 hrs. each (2 runs mixed). Temperature of Electrolysis: l,000 C.

Bath Gone; 10" depth, 2% by wt. calc. as W in CaCI Cathode: 2" dia. graphite, 7' immersion.

Anode: Graphite cell pot.

Cathode current density: 4.1 amps/in. calc.

Voltage: 2.2 volts.

Current: 180 amperes.

Screen Analysis:

Although the scheelite concentrate used in the feed con- The above mentioned apparatus was used to electrotained 4.5 wt. percent Fe and 3.5% M0, for some reason deposit molybdenum from crude CaMoO This crude the cleaned electrodeposite from av regulated bath confeed was provided by roasting molybdenite (M05 contained less than 0.25% Fe and less than .01% Mo. A centrates in the common commercial fashion. The raw possible explanation is that a CaCl electrolyte bearing material contained 48% Mo(98% (321M00 The conabout 10% by wt. of solute calculated as W salts out ditions of electrolysis were identical to those run for or precipitates CaMoO so that very little of any of the tungsten. The results are shown in Table V.

TABLE V Screen Analysis:

'ze +100 +150 +200 +325 325 Wt. 2. 0 15. 6 32. 7 40. 6 9.1 Principal Impurity, Fe 2. 1 2. 7 2. 2 2. 7 3.0

molybdenum is available for electrolysis. There is no The molybdenum metal deposited was silvery bright question that the impurities are found in the sludge which in color and free from carbides, lower oxides, and the forms in the bottom of the bath. The use of chlorine like. The iron content of the deposit was high due to promotes the separation. the corrosition of the pot lining, the corrosion products Inafreshlypreparedba-th, the chlorine treatment speeds falling into the bath. Normally, unless the bath is the time when the deposit fulfills required specifications. 40 treated with chlorine gas prior to electrolysis, the first Otherwise the first deposits are usually discarded. The dep ositsare not as pure as subsequent deposits. The M0 purity of the deposit improves with the age of the bath. in this example analyzed 97.0% for the reasons given. However, after prolonged use the impurities build up In order to provide an electrolyte capable of producing in volume so that it :becomes diificult not to physically molybdenum consistently having a purity in a bulk sample entrain impurities despite the chlorine treatment. At of 99.5% and selected crystals of 99.9%, chlorine gas is this point it is practical to ladle out the insolubles or bubbled through the electrolyte of the foregoing runs set start anew. forth in Table V until free chlorine is detected in the The preferred cathode material is graphite, but other efiluent gas stream. Also, as in connection with the inert electrical conductors maybe used. tungsten, molybdenum may be used as an anode in an Although the large bulk of the market is for 98.8% electrorefining process similar to the one used in connec- W, where product whose it i over 99 99% W i tion with the electrorefinin-g of tungsten to produce a required, the tungsten hereinabove described may be pure molybdenum metal. Similarly, as in the case of tungelectrorefined using the electrorefining method describd sten, a molybdenum chloride is formed in the electrolyte in my Patent No. 2,960,451 or treated in the following which i pr f r ly of a o five w ight percen way. The scope of the invention as described in the fore- The electrodeposit stripped in accordance with the foregoing is Set forth in the pp Claimsgoing procedure is fed to a cell adequately covered, 1 laimi airtight and under inert gas pressure wh it Serves as 1. In aprocess for the continuous production of tungsten anode. An inert cathode is immersed in the electrolyte metal in eXCeSS 0f 99% P y y the fusion electrolysis of for electrorefining of the tungsten, The preferred l t scheelite concentrates added to a calcium chloride eleclyte is a 5050 mol percent NaCl=KC1 solvent containtrolyte, improvement Comprising Controlling t e coming 3-5 wt. percent WCl The WCl solute is easily POSitiOH said electrolyte y bubbling chlorine gas generated within th 11 :by bubbling C1 thrgugh th through said electrolyte to convert to calcium chloride bed of tungsten resting on th botto of the ell, Th the calcium oxide released by the deposition of tungsten, anode current density is preferably below 10 amps/in. 55 thereby maintaining the high q y Production of 8- and l-3 amps/in. is adequate. The cathode current Sign at a high recovery efiiciencydensity may be ri d widely ith d results f a 2. A continuous process for the electrolytic production few tenths of an ampere per sq. in. to over 35 amps./in. of refractory mfiial 0f the group tungsten and y The preferred cathode current density is in the range of dfimlm comprising Providing a molten electiloiyte (10m- 3-10 amps./in. The temperature of electrolysis is po d of the 'group of alkaline earth metal halides and 750-800 C. The cathode and its electrodeposit are prefat l a t n OXidB 0f the group of alkaline earth metal erably cooled in an inert atmosphere. The adherent salts 6 8 in an ial amount in said electrolyte, adding are readily washed away in water. an oxide ore of said refractory metal including an oxide The tungsten metal crystals are macroscopic, silvery of the group of alkaline earth metals to maintain the white, ductile and of extraordinarily high purity in excess 7 desired amount of alkaline metal oxide in the electrolyte, of 99.99%. electrolyzing the said molten electrolyte between an anode and cathode for depositing said refractory metal in pure, ductile and large crystalline form on a cathode immersed in the electrolyte and adding chlorine to convert to alkaline earth metal chloride the oxide of the alkaline earth metal released by the deposition of the refractory metal, thereby maintaining the electrolyte at a high recovery efiiciency.

3. A continuous process for the electrolytic production of a refractory metal of the group tungsten and molybdenum as set forth in claim 2 wherein said alkaline earth halide is calcium chloride and said oxide ore is a scheelite concentrate for the production of pure ductile tungsten in large crystalline form.

4. A continuous process for the electrolytic production of a refractory metal of the group tungsten and molybdenum as forth in claim 2 wherein said refractory metal is molybdenum and said alkaline earth halide is calcium chloride.

5. In a process for the continuous production of molybdenum metal in excess of 99% purity by the fusion References Cited by the Examiner UNITED STATES PATENTS 2,960,451 11/1960 Slatin 204--64 JOHN H. MACK, Primary Examiner.

20 H. S. WILLIAMS, Assistant Examiner. 

2. A CONTINUOUS PROCESS FOR THE ELECTROLYTIC PRODUCTION OF A REFRACTORY METAL OF THE GROUP TUNGSTEN AND MOLYBDENUM COMPRISING PROVIDING A MOLTEN ELECTROLYTE COMPOSED OF THE GROUP OF ALKALINE EARTH METAL HALIDES AND AT LEAST ONE OXIDE OF THE GROUP OF ALKALINE EARTH METAL OXIDES IN SUBSTANTIAL AMOUNT IN SAID ELECTROLYTE, ADDING AN OXIDE ORE OF SAID REFRACTORY METAL INCLUDING AN OXIDE OF THE GROUP OF ALKALINE EARTH METALS TO MAINTAIN THE DESIRED AMOUNT OF ALKALINE METAL OXIDE IN THE ELECTROLYTE, ELECTROLYZING THE SAID MOLTEN ELECTROLYTE BETWEEN AN ANODE AND CATHODE FOR DEPOSITING SAID REFRACTORY METAL IN PURE, DUCTILE AND LARGE CRYSTALLINE FORM ON A CATHODE IMMERSED IN THE ELECTROLYTE AND ADDING CHLORINE TO CONVERT TO ALKALINE EARTH METAL CHLORIDE THE OXIDE OF THE ALKALINE EARTH METAL RELEASED BY THE DEPOSITION OF THE REFRACTORY METAL, THEREBY MAINTAINING THE ELECTROLYTE AT A HIGH RECOVERY EFFICIENCY. 